Method, apparatus and computer-readable code for detecting an incipient ground fault in an electrical propulsion system

ABSTRACT

Method, apparatus and computer-readable code for detecting an incipient ground fault. The apparatus includes a sensor for sensing a ground leakage signal. The apparatus further includes a correlator that receives the ground leakage signal, and at least one signal indicative of a respective energization state of a respective electrical phase for each of a plurality of electrical devices. The signal indicative of the respective energization state is modified during a time interval when a predetermined rate of change of voltage is expected. This rate of change is sufficient to cause an effect in the ground leakage signal. The correlator is configured to correlate the ground leakage signal with each received phase energization state signal, and supply a correlation signal. A processor is coupled for processing each of the individual correlation signals and extracting a correlation value from the supplied correlation signals that may be indicative of an incipient ground fault.

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 11/385,309 filed Mar. 21, 2006 now U.S. Pat. No. 7,248,057.This application is further related to U.S. patent application Ser. Nos.11/762,163 and 11/762,216, filed concurrently herewith. Each of theforegoing applications is incorporated by reference herein in theirentirety.

FIELD OF THE INVENTION

The present invention is generally related to electrical propulsionsystems, and more particularly, to a method, apparatus andcomputer-readable code for detecting an incipient ground fault that mayoccur in an electrical propulsion system of a traction vehicle.

BACKGROUND OF THE INVENTION

Locomotives and transit vehicles, as well as other large tractionvehicles used for heavy haul applications (off-highway trucks), commonlyuse an electrical propulsion system that includes various high powerelectrical components, such as generators, rectifiers, converters,traction motors, dynamic braking grids, cooling blowers, and the like.These components may fail over time due to various reasons, one of thembeing electrical grounds that may be caused by insulation degradation.For example, locomotives may operate in environments subject to varyingconditions, such as causative of freezing and thawing, which can degradean exposed electrical insulation, resulting in cracks.

The propulsion system of a locomotive has many insulated windings, andexcessive leakage current could develop over time due to variousfactors, such as aging, moisture, abrasions, dirt built-up and the like.This is especially true for the traction motors since moisture oftengets into these components because of their location and exposure torelatively harsh environmental conditions. Failures due to excessiveelectrical leakage currents in an electrical system of locomotives are aleading cause of system shutdowns and locomotive mission failures,similar problems exist in the above-noted vehicles.

Leakage current detectors have been used in many kinds of electricalequipment to protect the equipment from damage that could arise in thepresence of a large electrical current and/or to protect personnel frominjury, and there may be substantial industrial background on leakagecurrent monitoring by techniques used in electrical utility orindustrial applications. Ground faults may occur as a result of a faultin any of a number of different system components. In the context of alocomotive, such components by way of example can include the propulsiondrive system, batteries, and auxiliary equipment. Within the propulsiondrive system, ground faults can occur in one or several components,which include generator, rectifier, cabling, traction motor, dynamicbrake resistor, and blower motor.

A known difficulty in dealing with ground conditions in a locomotive isthat many of such conditions may be transitory in nature. Often when aground fault condition occurs, the affected portion of the electricalsystem is deactivated, and the locomotive is scheduled for repairs.However, once the locomotive is shopped for repairs, the system may nolonger exhibit abnormal grounds and the maintenance personnel cannotidentify the source of the fault. This is often because the excessiveleakage current is caused by moisture in the electrical components. Bythe time the locomotive is shopped, the moisture has dried out, thuseliminating the high leakage currents. The amount of moisture that isable to penetrate the insulation system and result in high leakagecurrents often depends in part on the condition of the insulationsystem. A properly operating system experiences relatively small changein leakage current as a result of changing moisture conditions, whereasa system with degraded insulation may experience large changes inleakage current that is moisture dependent.

In view of the foregoing considerations, it is desirable to have earlywarning of leakage current development (i.e., an incipient ground fault)in the electrical propulsion of the locomotive so that action may betaken before there is a locomotive disabling failure. More particularly,it would be desirable that such an action enables continued operation ofthe locomotive propulsion system so that the locomotive can fulfill amission and be able to return on its own power to a locomotive serviceshop for a thorough check and repair.

It would be further desirable to have the ability to determine in realtime the specific equipment that causes the incipient ground fault, asthat condition occurs, so that service personnel can retrieve at a latertime that information and be able to focus on that specific equipmentonce the locomotive eventually arrives at the service shop, withouttrying to mimic the environmental conditions that may have contributedto the occurrence of the incipient ground fault or without having tospend valuable resources and time on troubleshooting fault-freeequipment.

BRIEF DESCRIPTION OF THE INVENTION

The present invention in one aspect thereof provides an apparatus fordetecting an incipient ground fault that may occur in an electricalpropulsion system of a traction vehicle. The electrical propulsionsystem includes a plurality of electrical devices, each individualelectrical device being potentially subject to an incipient ground faultin at least one electrical phase thereof. The apparatus may include asensor configured to sense a respective ground leakage signal associatedwith a ground of the propulsion system. The apparatus may furtherinclude a correlator coupled to the sensor to receive the ground leakagesignal, and further coupled to receive at least one signal indicative ofa respective energization state of a respective electrical phase foreach respective one of the plurality of electrical devices. The at leastone signal indicative of the respective energization state of arespective electrical phase is configured to be modified during a timeinterval when a predetermined rate of change of voltage is expected.This predetermined rate of change of voltage is sufficient to cause aneffect in the characteristics of the ground leakage signal. Thecorrelator is configured to individually correlate the ground leakagesignal with each received phase energization state signal, and supply anindividual correlation signal between the ground leakage signal and eachreceived phase energization state signal. A processor is coupled to thecorrelator for processing each of the individual correlation signalssupplied by the correlator and extract a correlation value from thesupplied correlation signals. The correlation value may be indicative ofan incipient ground fault, if any, in a respective phase of one of theplurality of electrical devices.

The present invention in another aspect thereof provides a method fordetecting an incipient ground fault that may occur in an electricalpropulsion system of a traction vehicle. The electrical propulsionsystem includes a plurality of electrical devices, each individualelectrical device being potentially subject to an incipient ground faultin at least one electrical phase thereof. The method allows sensing arespective ground leakage signal associated with a ground of thepropulsion system. The method further allows monitoring at least onesignal indicative of a respective energization state of a respectiveelectrical phase for each respective one of the electrical devices andmodifying such at least one signal indicative of the respectiveenergization state during a time interval when a predetermined rate ofchange of voltage is expected. This predetermined rate of change ofvoltage is sufficient to cause an effect in the characteristics of theground leakage signal. The ground leakage signal is individuallycorrelated with each modified phase energization state signal. Anindividual correlation signal is generated between the ground leakagesignal and each modified phase energization state signal, and acorrelation value is determined from the generated correlation signals.The correlation value may be indicative of an incipient ground fault, ifany, in a respective phase of one of the plurality of electricaldevices.

The present invention in yet another aspect thereof provides an articleof manufacture comprising a computer program product comprising acomputer-usable medium having a computer-readable code therein fordetecting an incipient ground fault that may occur in an electricalpropulsion system of a traction vehicle. The electrical propulsionsystem includes a plurality of electrical devices, each individualelectrical device being potentially subject to an incipient ground faultin at least one electrical phase thereof. The computer-readable codeincludes: computer-readable code responsive to a sensed ground leakagesignal associated with a ground of the propulsion system;computer-readable code for monitoring at least one signal indicative ofa respective energization state of a respective electrical phase foreach respective one of the electrical devices; computer-readable codefor modifying the at least one signal indicative of the respectiveenergization state during a time interval when a predetermined rate ofchange of voltage is expected. This predetermined rate of change ofvoltage is sufficient to cause an effect in the characteristics of theground leakage signal; computer-readable code for individuallycorrelating the ground leakage signal with each modified phaseenergization state signal; computer-readable code for generating anindividual correlation signal between the ground leakage signal and eachmodified phase energization state signal; and computer-readable code fordetermining a correlation value from the generated correlation signals.The correlation value may be indicative of an incipient ground fault, ifany, in a respective phase of one of the plurality of the electricaldevices.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages of the invention will be more apparent fromthe following description in view of the drawings that show:

FIG. 1 is a schematic of one exemplary locomotive propulsion system;

FIG. 2 is a schematic of another exemplary locomotive propulsion system;

FIG. 3 illustrates circuit details regarding an exemplary ground faultdetector embodying aspects of the present invention;

FIGS. 4-6 show graphical plots of exemplary test results of an apparatusfor detecting an incipient ground fault in accordance with aspects ofthe present invention;

FIGS. 7 and 8 show example waveforms as may develop in a prior artapparatus for detecting an incipient ground fault; and

FIGS. 9 and 10 show example waveforms as may develop in an apparatusembodying aspects of the present invention for detecting an incipientground fault that may occur in an electrical propulsion system of atraction vehicle.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified schematic of one exemplary propulsion system in atraction vehicle, such as a locomotive, a transit vehicle, or any otherlarge traction vehicle as may be used for heavy haul applications (e.g.,off-highway trucks). In one example embodiment, the propulsion systemincludes a plurality of electrical devices that individually may bepotentially subject to an incipient ground fault in at least oneelectrical phase thereof. Examples of such devices may include analternator 24, a main rectifier 26, and a dynamic braking grid 28, whichmay include blowers 30. Further, illustrated in FIG. 1 are six tractionmotors 40, 42, 44, 46, 48, and 50, where each motor may be electricallyconnected to a respective inverter 52. As illustrated in FIG. 2 inconnection with another example of a locomotive propulsion system 20,the electrical devices may further comprise various other electricaldevices, such as various types of auxiliary electrical loads 54, energystorage devices 56 (e.g., batteries) as may be used in a hybrid vehicle,DC voltage step-up/step-down devices 58 (e.g., choppers), etc.

Returning to FIG. 1, the propulsion system may further include apropulsion system controller 60, a traction motor controller 62, and aground fault detector 70 embodying aspects of the present invention. Byway of example, a ground fault detector embodying aspects of the presentinvention will enable to detect essentially in real time (e.g., in theorder of milliseconds) and with a single sensor the location of anincipient ground fault (i.e., a leakage current which if not quicklyinterrupted could result in a vehicle-disabling failure). The detectiontechnique is performed without interrupting operation of the electricaldevices that make up the propulsion system, and consequently there isvirtually no degradation in the operational performance of thepropulsion system.

FIG. 3 illustrates details regarding an exemplary ground fault detector70 that may include a sensor 72 configured to sense a respective groundleakage signal associated with a ground of the propulsion system. In oneexample, sensor 72 may comprise a current sensor 74 connected at anelectrically neutral node 76, and may further comprise one or morevoltmeters, such as a voltmeter 78 connected between a positive side 80of a direct current (DC) bus and node 76, and/or a voltmeter 82connected across the positive side 80 and a negative side 84 of the DCbus.

A correlator 86 is coupled to the sensor 72 to receive the groundleakage signal, and is further coupled to receive at least one signal 88indicative of a respective energization state of a respective electricalphase for each respective one of the electrical devices. Each signal 88is configured to be modified during a time interval when a predeterminedrate of change of voltage is expected. This predetermined rate of changeof voltage is sufficient to cause an effect in the characteristics ofthe ground leakage signal (e.g., may cause a spike in the magnitude ofthe leakage current). In one example embodiment the modificationcomprises masking (e.g., zeroing) each signal 88 during the timeinterval when the high rate of change of voltage is expected, such asduring a transition from one energization state to another energizationstate. For example, transitioning from a positive voltage (e.g., +700 V)present in the positive side of the DC bus to a negative voltage (−700V) present in the negative side of the DC bus or vice versa.

FIG. 7 is a plot of example waveforms as may develop in a prior artapparatus for detecting an incipient ground fault that may occur in anelectrical propulsion system of a traction vehicle. Waveform 120represents voltage across a respective phase winding and waveform 124represents ground leakage current within acceptable levels in FIG. 7,and showing an incipient ground fault in FIG. 8. Note that during timeintervals 126 and 128 when the predetermined rate of change of voltageoccurs (e.g., a relatively high rate of change of voltage) in waveform120, there is a noticeable effect on the characteristics of the groundleakage current, which can impair accurate and/or consistent detectionof the incipient ground fault. As will be appreciated by one skilled inthe art, these transient spikes can have a significantly highermagnitude of current relative to the magnitude of the leakage current(when the transients are not present), and thus, even a relatively smallpercentage error that can be introduced while processing the leakagecurrent during time intervals 126 and 128, can result in an unacceptablelevel of uncertainty for detecting the incipient ground fault.

FIG. 9 is a plot of example waveforms as may develop in apparatusembodying aspects of the present invention for detecting an incipientground fault that may occur in an electrical propulsion system of atraction vehicle. Waveform 220 represents voltage across a respectivephase winding and waveform 224 represents ground leakage current, withinacceptable levels in FIG. 9, and showing an incipient ground fault inFIG. 10. Note that during time intervals 226 and 228 when thepredetermined rate of change of voltage (high dv/dt) is expected inwaveform 220, a masking action performed in accordance with aspects ofthe present invention allows removing undesirable effects on thecharacteristics of the ground leakage current, and this advantageouslyresults in a more accurate and/or consistent detection of the incipientground fault. For example, it is seen that in FIG. 9 and in FIG. 10 thatduring time intervals 226 and 228, the leakage current is free fromspikes that can impair accurate and/or consistent detection of the priorart apparatus for detecting an incipient ground fault.

Each signal 88 provides an indication of whether the electrical phase ofa respective one of the electrical devices is connected to a respectiveside of the DC bus. For example, block 1 _(A) represents a signalindicative of the respective energization state of a first electricalphase (e.g., phase A) for a first one of the electrical devices. In thisexample, block 1 _(B) represents a signal indicative of the respectiveenergization state of a second electrical phase (e.g., phase B) for thefirst of the electrical devices and block 1 _(C) represents a signalindicative of the respective energization state of a third electricalphase (e.g., phase C) for the first of the electrical devices.Similarly, block n_(A) represents a signal indicative of the respectiveenergization state of a first electrical phase (e.g., phase A) for annth one of the electrical devices. In this example, block n_(B)represents a signal indicative of the respective energization state of asecond electrical phase (e.g., phase B) for the nth of the electricaldevices and and block n_(C) represents a signal indicative of therespective energization state of a third electrical phase (e.g., phaseC) for the nth of the electrical devices. Examples of a signal 88 may bea phase firing signal such as the firing signal applied to a powerswitching device, such as an Insulated Gate Bipolar Transistor (IGBT) ina given inverter 52, or may be a voltage signal, such as the voltagethat is developed across the phase of a given electrical device withrespect to the positive (or the negative) side of the DC bus connectedto the inverter, or may be a phase current signal, such as the currentthat flows through the phase of a given electrical device, or may be thecurrent that flows through the switching device such as an IGBT, or maybe a combination of the foregoing signals. For example in the case of adiode rectifier 26 connected between the alternator and the DC bus, thestate of diode conduction (and hence whether the phase is connected tothe positive or negative side of the DC bus) may be determined from thedirection of current flowing in the phase. Similarly, when there iscurrent flowing in the positive A phase diode, for example, then thisindicates that phase A is connected to DC positive bus.

Correlator 86 is configured to individually correlate the ground leakagesignal with each received phase energization state signal 88, and supplyan individual correlation (e.g., temporal interdependence) signalbetween the ground leakage signal and each received phase energizationstate signal. One example of correlator 86 is a multiplier device. Thatis, a device that multiplies the ground leakage signal from sensor 72with each received phase energization state signal 88. In one exampleembodiment the multiplier value is set to zero during the time intervalwhen the predetermined rate of change of voltage is expected, such asduring a transition from one energization state to another energizationstate. Outside this time interval the multiplier value may be equal tounity, and may include a positive or a negative sign depending on theapplicable energization state, as indicated in the examplerepresentations of energization state signal 88 shown in FIG. 3. It willbe appreciated that setting the multiplier value to zero during the timeinterval when the predetermined rate of change of voltage is expected isone example of how one can perform the masking action that allows toremove the relatively high magnitude spikes that can impair accurateand/or consistent detection of the prior art apparatus for detecting anincipient ground fault.

A filter 90, such a low pass filter, may be coupled to the correlator 86to provide a predefined filtering to the supplied correlation signals.This would enable to extract a DC (or low frequency) content from thecorrelation signal, such as may be present in an incipient DC faultground. A processor 92 may be coupled to the filter 90 for processingeach of the filtered correlation signals and extract a correlation valuefrom the filtered correlation signals. The correlation value to beextracted may be determined based on a comparison of the respectivevalues of the correlation signals with respect to a predefined thresholdvalue. For example, the extracted value may correspond to thecorrelation signal that exceeds the threshold value. This extractedcorrelation value, if any, is indicative of an incipient ground fault ina respective phase of one of the plurality of electrical devices. Thatis, this extracted value would indicate a strong temporal (time)interdependence or association between the ground leakage signal and themoment in time when a given electrical phase of a given electricaldevice is connected to the DC bus. It is noted that, as the correlationoperation is performed, a temporarily shift in frequency or phase may beapplied to the signals indicative of the respective energization stateof the electrical phases of a given electrical device. This would allowlocating ground faults that may occur in different phases of the sameelectrical device. Otherwise such multiple faults could be masked dueto, for example, synchronicity between the phases of the firing signals.

It is noted that for the sake of simplicity of illustration FIG. 3 justshows one correlator and one low pass filter associated with a singlesignal 88. In the general case, assuming a number of m signals 88 (wherem is a positive integer larger than one) one would employ a number of mcorrelators and m low pass filters, in lieu of the single correlator andfilter shown in FIG. 3. For example, assuming six three-phase electricaldevices, then m=6×3=18.

FIGS. 4-6 show graphical plots as a function of time of exemplary testresults of an apparatus for detecting an incipient ground fault inaccordance with aspects of the present invention. More particularly,FIG. 4 illustrates a non-fault ground condition in the propulsion systemwhere, for example, a signal plot 102 (left vertical scale) correspondsto the ground leakage signal and a signal plot 104 (right verticalscale) corresponds to a correlated output signal.

By way of comparison, FIG. 5 illustrates an incipient ground fault beingdetected in a given electrical phase of a given electrical device. Forexample, the signal plot 106 (left vertical scale) illustrates a DCcontent in the ground leakage signal and a signal plot 108 (rightvertical scale) illustrates a fast and measurable transient (in theorder of a second or less, thereby demonstrating a desirablecharacteristic of a detector embodying aspects of the present invention)in the correlated output signal.

FIG. 6 illustrates exemplary signal plots in a given electrical deviceduring the presence of an incipient ground fault in the electrical phaseof another electrical device. A signal plot 110 (left vertical scale)illustrates a ground leakage signal similar to the one shown in FIG. 4during a no ground fault condition. Similarly, a signal plot 112 (rightvertical scale) for the correlated output signal of the electricaldevice without the incipient ground fault shows no evidence of a faultcondition, thereby demonstrating that an apparatus for detecting anincipient ground fault in accordance with aspects of the presentinvention is substantially impervious to cross-coupling effects due toground fault conditions present in other electrical devices whilemonitoring a given electrical device.

Aspects of the present invention can be embodied in the form ofcomputer-implemented processes and apparatus for practicing thoseprocesses. The present invention can also be embodied in the form ofcomputer program code including computer-readable instructions embodiedin tangible media, such as floppy diskettes, CD-ROMs, hard drives, orany other computer-readable storage medium, wherein, when the computerprogram code is loaded into and executed by a computer, the computerbecomes an apparatus for practicing the invention. When implemented on acomputer, the computer program code segments configure the computer tocreate specific logic circuits or processing modules.

While the preferred embodiments of the present invention have been shownand described herein, it will be obvious that such embodiments areprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those of skill in the art without departingfrom the invention herein. Accordingly, it is intended that theinvention be limited only by the spirit and scope of the appendedclaims.

1. Apparatus for detecting an incipient ground fault that may occur inan electrical propulsion system of a traction vehicle, said electricalpropulsion system comprising a plurality of electrical devices, eachindividual electrical device being potentially subject to an incipientground fault in at least one electrical phase thereof, said apparatuscomprising: a sensor configured to sense a ground leakage signalassociated with a ground of the propulsion system; a correlator coupledto the sensor to receive the ground leakage signal, and further coupledto receive at least one signal indicative of a respective energizationstate of a respective electrical phase for each respective one of theplurality of electrical devices, wherein said at least one signalindicative of the respective energization state of a respectiveelectrical phase is configured to be modified during a time intervalwhen a predetermined rate of change of voltage is expected, saidpredetermined rate of change of voltage being sufficient to cause aneffect in the characteristics of the ground leakage signal, saidcorrelator configured to individually correlate the ground leakagesignal with each received phase energization state signal and supply anindividual correlation signal between the ground leakage signal and eachreceived phase energization state signal; and a processor coupled to thecorrelator for processing each of the individual correlation signalssupplied by the correlator and extracting a correlation value from thesupplied correlation signals, said correlation value being indicative ofan incipient ground fault, if any, in a respective phase of one of theplurality of electrical devices.
 2. The apparatus of claim 1 wherein themodification of said at least one signal comprises masking said at leastone signal during the time interval.
 3. The apparatus of claim 1 whereinthe expectation of the predetermined rate of change of voltagecorresponds with a transition from one energization state to anotherenergization state.
 4. The apparatus of claim 1 wherein the signalindicative of the respective energization state of the electrical phaseof a respective electrical device is selected from the group consistingof a phase firing signal, a voltage signal with respect to a directcurrent (DC) bus, a phase current signal, a current that flows through apower switching device connected to the DC bus, and a combination of theforegoing signals.
 5. The apparatus of claim 1 further comprising a lowpass filter connected to the correlator and configured to provide apredefined filtering to the supplied correlation signals.
 6. Theapparatus of claim 1 wherein the sensor configured to sense therespective ground leakage signal is selected from the group consistingof a current sensor, and at least one or more voltage sensors.
 7. Theapparatus of claim 1 wherein the traction vehicle is selected from thegroup consisting of a locomotive, a transit vehicle, and an off-highwayvehicle.
 8. The apparatus of claim 1 wherein the correlator comprises amultiplier.
 9. The apparatus of claim 1 wherein said correlatorcomprises a plurality of m correlators, each of said m correlators beingindividually connected to receive one signal indicative of a respectiveenergization state of a respective electrical phase of one of theplurality of electrical devices, wherein m is a positive integercorresponding to the number of electrical devices times the number ofelectrical phases per device.
 10. The apparatus of claim 5 wherein saidcorrelator comprises a plurality of m correlators, each of said mcorrelators being individually connected to receive one signalindicative of a respective energization state of a respective electricalphase of one of the plurality of electrical devices, wherein m is apositive integer corresponding to the number of electrical devices timesthe number of electrical phases per device, and further wherein said lowpass filter comprises a plurality of m low pass filters, each one ofsaid m filters being individually connected to one of said m correlatorsto provide a predefined filtering to one supplied correlation signal.11. A method for detecting an incipient ground fault that may occur inan electrical propulsion system of a traction vehicle, said electricalpropulsion system comprising a plurality of electrical devices, eachindividual electrical device being potentially subject to an incipientground fault in at least one electrical phase thereof, said methodcomprising: sensing a ground leakage signal associated with a ground ofthe propulsion system; monitoring at least one signal indicative of arespective energization state of a respective electrical phase for eachrespective one of the electrical devices; modifying said at least onesignal indicative of the respective energization state during a timeinterval when a predetermined rate of change of voltage is expected,said predetermined rate of change of voltage being sufficient to causean effect in the characteristics of the ground leakage signal;individually correlating the ground leakage signal with each modifiedphase energization state signal; generating an individual correlationsignal between the ground leakage signal and each modified phaseenergization state signal; and determining a correlation value from thegenerated correlation signals, said correlation value being indicativeof an incipient ground fault, if any, in a respective phase of one ofthe plurality of electrical devices.
 12. The method of claim 11 whereinthe modifying of said at least one signal comprises masking said atleast one signal during the time interval.
 13. The method of claim 11wherein the expectation of the predetermined rate of change of voltagecorresponds with a transition from one energization state to another.14. An article of manufacture comprising a computer program productcomprising a computer-usable medium having a computer-readable codetherein for detecting an incipient ground fault that may occur in anelectrical propulsion system of a traction vehicle, said electricalpropulsion system comprising a plurality of electrical devices, eachindividual electrical device being potentially subject to an incipientground fault in at least one electrical phase thereof, saidcomputer-readable code comprising: computer-readable code responsive toa sensed ground leakage signal associated with a ground of thepropulsion system; computer-readable code for monitoring at least onesignal indicative of a respective energization state of a respectiveelectrical phase for each respective one of the electrical devices;computer-readable code for modifying said at least one signal indicativeof the respective energization state during a time interval when apredetermined rate of change of voltage is expected, said predeterminedrate of change of voltage being sufficient to cause an effect in thecharacteristics of the ground leakage signal; computer-readable code forindividually correlating the ground leakage signal with each modifiedphase energization state signal; computer-readable code for generatingan individual correlation signal between the ground leakage signal andeach modified phase energization state signal; and computer-readablecode for determining a correlation value from the generated correlationsignals, said correlation value being indicative of an incipient groundfault, if any, in a respective phase of one of the plurality of theelectrical devices.
 15. The article of manufacture of claim 14 whereinthe computer-readable code for modifying said at least one signalcomprises computer-readable code for masking said at least one signalduring the time interval.
 16. The article of manufacture of claim 14wherein the expectation of the predetermined rate of change of voltagecorresponds with a transition from one energization state to anotherenergization state.